This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Atrial fibrillation is a medical condition in which patients have a rapid and irregular heart beat. The rapid and irregular heart beat leads to poor performance of the heart especially in the left atrium. Observational cohort studies have shown that atrial fibrillation increases the risk of stroke and heart failure. In patients with poor control of heart rate and symptoms, despite adequate medical treatment, a catheter ablation procedure is recommended. While the left atrial ablation procedure has been successful in preventing the atrial fibrillation recurrence compared to medication therapy, the effect of the ablation procedure on left atrial performance is not well studied. Most previous studies have looked at atrial fibrillation recurrence as an end point, but not left atrial performance. While excessive ablation could cause damage to the left atrium and surrounding structures, optimizing ablation therapy to improve left atrial performance and decrease atrial fibrillation recurrence would be ideal. A decreased left atrial performance may lead to poor functional ability and a long-term increased risk of stroke. Our aim is to conduct a pilot study to determine the variance in measurements of left atrial performance before and after catheter ablation, using a three-dimensional magnetic resonance imaging of the heart during stress induced by treadmill exercise. Our idea to study the treadmill exercise induced changes in left atrial performance is novel. We also propose to determine the variances in measurements of functional ability before and after catheter ablation.